G-CAT is a covariance analysis tool that enables fast and accurate computation of error ellipses for descent, landing, ascent, and rendezvous scenarios, and quantifies knowledge error contributions needed for error budgeting purposes. Because GCAT supports hardware/system trade studies in spacecraft and mission design, it is useful in both early and late mission/ proposal phases where Monte Carlo simulation capability is not mature, Monte Carlo simulation takes too long to run, and/or there is a need to perform multiple parametric system design trades that would require an unwieldy number of Monte Carlo runs.

Landing Site Reconstruction based on descent camera imagery. Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.
G-CAT is formulated as a variable-order square-root linearized Kalman filter (LKF), typically using over 120 filter states. An important property of G-CAT is that it is based on a 6-DOF (degrees of freedom) formulation that completely captures the combined effects of both attitude and translation errors on the propagated trajectories. This ensures its accuracy for guidance, navigation, and control (GN&C) analysis. G-CAT provides the desired fast turnaround analysis needed for error budgeting in support of mission concept formulations, design trade studies, and proposal development efforts.

The main usefulness of a covariance analysis tool such as G-CAT is its ability to calculate the performance envelope directly from a single run. This is in sharp contrast to running thousands of simulations to obtain similar information using Monte Carlo methods. It does this by propagating the “statistics” of the overall design, rather than simulating individual trajectories.

G-CAT supports applications to lunar, planetary, and small body missions. It characterizes onboard knowledge propagation errors associated with inertial measurement unit (IMU) errors (gyro and accelerometer), gravity errors/dispersions (spherical harmonics, masscons), and radar errors (multiple altimeter beams, multiple Doppler velocimeter beams). G-CAT is a standalone MATLAB- based tool intended to run on any engineer’s desktop computer.

This work was done by Dhemetrios Boussalis and David S. Bayard of Caltech for NASA’s Jet Propulsion Laboratory.

This software is available for commercial licensing. Please contact Dan Broderick at This email address is being protected from spambots. You need JavaScript enabled to view it.. NPO-47854

Topics:
Aircraft instruments Altimeters Analysis methodologies Avionics Computer software and hardware Electronic equipment Entry, descent and landing Entry, descent, and landing Hardware Information Technology Parts Radar Simulation and modeling Software Spacecraft Statistical analysis Tools and equipment Vehicles
This Brief includes a Technical Support Package (TSP).
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Covariance Analysis Tool (G-CAT) for Computing Ascent, Descent, and Landing Errors

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    Software Tech Briefs Magazine

    This article first appeared in the September, 2013 issue of Software Tech Briefs Magazine (Vol. 37 No. 9).

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    Overview

    The document outlines the development and application of the Covariance Analysis Tool (G-CAT) at the Jet Propulsion Laboratory (JPL), aimed at enhancing guidance, navigation, and control (GN&C) capabilities for sample return missions, particularly focusing on lunar missions. Initially, the initiative targeted comet sample return missions, but it was redirected in 2010 to support lunar descent, landing, and ascent scenarios.

    The G-CAT is designed to address a critical need in early mission proposal development, where quick and accurate analysis of various GN&C configurations is essential. Traditional Monte Carlo simulation tools, which are often time-consuming and not available until later phases of mission development, pose challenges for early-stage analysis. G-CAT provides a solution by enabling fast computation of descent and ascent error ellipses, quantifying knowledge error contributions for error budgeting, and facilitating hardware/system trade studies.

    The tool operates by propagating statistical moments of the overall design rather than simulating individual trajectories, allowing for efficient performance envelope calculations from a single run. This capability significantly reduces the need for extensive Monte Carlo simulations, making it invaluable for both early and late mission phases. The document emphasizes that a single run of G-CAT can yield insights equivalent to thousands of Monte Carlo simulations, thereby streamlining the mission design process.

    The strategic focus of the initiative is to strengthen proposals for lunar sample return missions, such as the Moonrise Step 2 proposal to New Frontiers. The development effort is concentrated in two main areas: the GN&C covariance analysis tool itself and methods for reconstructing landing site locations using imagery from descent cameras.

    Overall, the document highlights the importance of G-CAT in improving the competitiveness of JPL in developing successful mission proposals by providing robust analytical support during the critical early stages of mission planning. The tool's ability to quickly assess various design configurations enhances the overall safety and effectiveness of future lunar and planetary exploration missions.